NAT'L INST. OF STAND & TECH

A111D7 231flflD

A UNITED STATES DEPARTMENT OF COMMERCE

PUBLICATION

NBS TECHNICAL NOTE 379-1

Standard Time and Frequency:

Its Generation, Control, and Dissemination

From the National Bureau of Standards

Time and Frequency Division
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards1 was established by an act of Congress March 3, 1901. The Bureau's overall goal is to strengthen and advance the Nation's science and

technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measure- ment system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety.

The Bureau consists of the Institute for Basic Standards, the Institute for Materials

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that system with measurement systems of other nations; and furnishes essential services leading to accurate and uniform physical measurements throughout the Nation's scientific community, industry, and commerce. The Institute consists of a Center for Radiation Research, an Office of Measurement Services and the following divisions: Applied Mathematics—Electricity—Heat—Mechanics—Optical Physics—Linac

Radiation2—Nuclear Radiation2—Applied Radiation2—Quantum Electronics3—

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. THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research lead- ing to improved methods of measurement, standards, and data on the properties of

well-characterized materials needed by industry, commerce, educational institutions, and

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and develops, produces, and distributes standard reference materials. The Institute consists of the Office of Standard Reference Materials and the following divisions: Analytical Chemistry—Polymers—Metallurgy—Inorganic Materials—Reactor

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and methods of test; and provides technical advice and services to Government agencies

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THE CENTER FOR COMPUTER SCIENCES AND TECHNOLOGY conducts re- search and provides technical services designed to aid Government agencies in improving cost effectiveness in the conduct of their programs through the selection, acquisition,

and effective utilization of automatic data processing equipment; and serves as the principal focus within the executive branch for the development of Federal standards for automatic data processing equipment, techniques, and computer languages. The Center

consists of the following offices and divisions: Information Processing Standards—Computer Information—Computer Services —Systems Development—Information Processing Technology.

THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination

and accessibility of scientific information generated within NBS and other agencies of

the Federal Government; promotes the development of the National Standard Reference

Data System and a system of information analysis centers dealing with the broader

aspects of the National Measurement System; provides appropriate services to ensure

that the NBS staff has optimum accessibility to the scientific information of the world,

and directs the public information activities of the Bureau. The Office consists of the following organizational units: Office of Standard Reference Data—Office of Technical Information and

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1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D.C. 20234.

2 Part of the Center for Radiation Research.

3 Located at Boulder, Colorado 80302.
Nat&iial Bursas $ Standards

MAY 1 3 1974 UNITED STATES DEPARTMENT OF COMMERCE

Maurice H. Stans, Secretary

NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director

c.c

NBS TECHNICAL NOTE 379-1

ISSUED JUNE 1972

Nat. Bur. Stand. (U.S.), Tech. Note 379-1, 26 pages (June 1972)

CODEN: NBTNAE

Standard Time and Frequency:

Its Generation, Control, and Dissemination

From the National Bureau of Standards

Time and Frequency Division

John B. Milton

Time and Frequency Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide

a means for making available scientific data that are

of transient or limited interest. Technical Notes may

be listed or referred to in the open literature.

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. (Order by SD Catalog No. C13.46:379-l). Price 35 cents. 20402

TABLE OF CONTENTS

Page

ABSTRACT 1

1. INTRODUCTION 1

2. STANDARD FREQUENCY AND TIME GENERATION 4

2. 1 Atomic Frequency and Time Standards Section 4

2.1.1 The NBS frequency standard 4

2. 1.2 Computed or "paper" time scales 5

2. 1.3 Operational clock systems 6

2.2 Radio Station WWV, Fort Collins, Colorado 7

2.3 Radio Station WWVB, Fort Collins, Colorado 7

2.4 Radio Station WWVL, Fort Collins, Colorado 8

2.5 Radio Station WWVH, Kauai, Hawaii 8

3. TRANSMISSION OF TIME AND FREQUENCY 9

3. 1 From WWV 9

3.2 From WWVB 9

3.3 From WWVL (Experimental) 10

3.4 From WWVH 11

4. TIME AND FREQUENCY INTERCOMPARISONS 11

4.1 WWV and WWVH Self-Comparisons 11

4.2 WWV to WWVB and WWVL 12

4.3 Between the Fort Collins Standards and the

WWVH Standard 12

4.4 Between the Fort Collins Standards and the

NBS Standards 13

4.4.1 Portable clocks 13

4.4. 2 TV synchronizing pulse method 13

a. Theory 13

b. Practic e 15

5. APPENDIX 15

5. 1 Explanation of NBS Time Scales . 15

5. 2 Figures 16-22

in
LIST OF FIGURES

Figure 1:

Figure 2:

Figure 3°

Figure 4:

Figure

Figure

Figure

5:

Page

System for Generation of Computed Time

Scales AT(NBS) and UTC(NBS) 16

Standard Frequency and Time Interval

Generating System, Time and Frequency

Division, Section 273.04 17

WWV and WWVH Time and Frequency

Generation- -One of Three Identical Systems

Showing Intercomparisons 18

WWVB Time and Frequency Generation and

Control- -One of Three Semi-independent

Systems, Fort Collins, Colorado 19

WWVL Frequency Generation and Control

System, Fort Collins, Colorado 20

Clock Intercomparison WWV-WWVB -WWVL,

Fort Collins, Colorado 21

Space -Time Diagram of Boulder to Fort Collins

Clock Synchronizing. Diagram at One Instant of

Time 22

Bulletin 100 May 1, 1972

BULLETIN

NBS Radio Station WWVL

Notice is hereby given that effective 0000 UTC 17 May 1972,

the WWVL broadcast format will be changed from 19. 9 kHz and

20. kHz to 20. kHz and 20. 9 kHz with each frequency transmitted on a 50% duty cycle.

Effective 1 July 1972, regularly scheduled transmissions from

WWVL will be discontinued. Contingent upon the need and the

availability of funds, the station will broadcast experimental programs thereafter on an intermittent basis only.

Transmissions may be made available on a subscription basis

to organizations or agencies of the Federal government. Arrangements for use of WWVL to broadcast experimental programs

should be made through the Frequency-Time Broadcast Services

Section 273. 02, National Bureau of Standards, Boulder, CO 80302.

IV
STANDARD TIME AND FREQUENCY: ITS GENERATION, CONTROL,

AND DISSEMINATION FROM THE NATIONAL BUREAU OF STANDARDS

TIME AND FREQUENCY DIVISION

John B. Milton

The Time and Frequency Division of the National Bureau

of Standards produces the NBS time scales, AT(NBS)

and UTC(NBS). These time scales are developed byutilizing the properties of the NBS frequency standard,

NBSFS. The main byproduct of these time scales is

the operational clock systems. These operational clock

systems are used, among other things, to calibrate the

clocks and secondary standards necessary for the operation of the NBS radio stations, WWV, WWVB, WWVL, and

WWVH. These stations transmit UTC(NBS), and various

tones, alerts, and corrections for time -of -day information.

Key Words: clock synchronization; frequency and time

dissemination; primary frequency standard;

standard frequency broadcasts; time interval; time scales

1. INTRODUCTION

Since its inception in 1901, the National Bureau of Standards has

been involved in the design, construction, maintenance, and improvement of standards of frequency and time interval. During the first two

decades of the Twentieth Century, the work on these standards was

done by quite diverse groups.

The people maintaining the standard of time interval were con- cerned primarily with problems of navigation, while the standard of

frequency was maintained by a group concerned chiefly with radio

interference between adjacent channels in the radio spectrum.

The operational time -interval standard has progressed from a pendulum clock with electrical pulse output through crystal clocks to

the present-day "atomic clock. " During the same period, the
operational standard of frequency has evolved from a tuning fork to a quartz crystal oscillator to an atomic beam device. While there may

be only a superficial connection between a pendulum clock and a tuning

fork, the present-day standards of time interval and frequency are, by

their very nature, inseparable.

Along with the maintenance and improvement of these standards

came the need to disseminate these standards. In 1923 the National

Bureau of Standards radio station WWV was established to disseminate

the standard of frequency via experimental broadcasts. Time pulses

were added to these broadcasts in 193 5. Voice announcements of the

time-of-day were added in 1945.

In 1948, NBS radio station WWVH was placed in operation. In

the 1950's, very good commercial standards were controlling both of

these high-frequency radio stations. At that time, nothing was done at

the stations beyond providing good crystal oscillators. Utilizing the

ultra-high precision oscillators located at the NBS Boulder Laboratories,

the time and frequency of these stations were measured daily. Any

deviations from the NBS standard were noted and corrective instructions

were relayed to the radio stations.

In 1956, the Boulder Laboratories of the NBS began experimental

transmissions on 60 kHz from radio station WWVB. The 60 kHz driving

frequency for this station was derived directly from the NBS standard of

frequency located at the Boulder Laboratories. There was no control of

the transmissions beyond providing this standard driving frequency.

In I960, utilizing an antenna borrowed from the Central Radio

Propagation Laboratory of NBS, radio station WWVL was placed in

experimental operation on 20 kHz. This station was located some ten

miles west of the Laboratories in a high mountain valley. Direct

generation of the transmission frequency by the Boulder Laboratories
was not feasible, but a method of phase control using servo systems

was employed. Since January 1, I960, when NBS began using a cesium

beam device as its official standard, all broadcast frequencies have

been referenced to the cesium resonance frequency.

WWVB and WWVL were moved to Fort Collins, Colorado, and

began operation there in July, 1963. The original time and frequency

control for these stations was accomplished by continuously operating

servo systems that compared the phase of these stations as received

in Boulder against a phase reference derived from the NBS Frequency

Standard. Phase corrective information was then sent to Fort Collins

via a radio link.

Primarily in order for NBS to increase the precision and

accuracy of the transmissions from station WWV, through more uniform U. S. coverage and the replacement of obsolete transmitting and

control equipment, a new facility for that station was installed at Fort

Collins, Colorado, in 1966.

Stations WWV, WWVB, and WWVL are, in essence, a composite facility with the ability to disseminate standards of time and

frequency from 19-9 kHz to 25 MHz. These stations provide standard

atomic frequency and a Universal Time scale, called UTC, which

approximates the UT1 scale based on the rotation of the earth.

Even though the time and frequency control of these stations

was provided by a highly precise ensemble of clocks and standards, the

problem remained of finding a simple, reliable, and inexpensive method

of providing a daily calibration of the Fort Collins standards with

respect to the NBS reference standards in Boulder.

This paper describes some of the current efforts of the Time

and Frequency Division of the NBS. These include the generation of

the computed NBS time scales, their translation into real working
clocks, and the use of these clocks in coordination efforts with other

standards laboratories. The coordination of the Fort Collins site

clocks and frequency standards, the measurement and control of LF

and VLF radiated phase, and the method of coordination of the Fort

Collins master clock with that of NBS/Boulder are also described.

2. STANDARD FREQUENCY AND TIME GENERATION

2. 1 Atomic Frequency and Time Standards Section

With the advent of the first working atomic clock system in

1948, developed by Harold Lyons at NBS/Washington; the era of the

highly accurate atomic standards was born. The first work at NBS/Boulder was under the project direction of Dr. R. C. Mockler.

This early work included, among other things, the refinement of the

original NBS cesium beam device, NBS-I.

In late 1959, the Atomic Frequency Standards Project,

including the cesium beam and ammonia maser development work, was

combined with the project concerned with theoretical and practical

aspects of atomic time scales to form the Atomic Frequency and Time

Standards Section.

2.1.1 The NBS Frequency Standard

Since about 1959 the NBS Frequency Standard (NBSFS) has been

based on a series of thoroughly -evaluated laboratory cesium beam

standards, designated NBS-I, NBS-II, and NBS-III. During this period

the specified one-sigma accuracy of the NBSFS — i. e., an estimate of

how far the output frequency of the standard may deviate from the

"ideal" frequency of the isolated cesium atom, has improved from

11 -13 several parts in 10 to 5 X 10 . The accuracy estimates are determined from an extensive set of evaluative measurements to assess
the magnitude of any possible frequency errors that might be introduced

by various components, subsystems, and environmental conditions of

the overall NBSFS system. Currently, the Atomic Frequency and Time

Standards Section is developing a new, refined cesium standard, designated NBS-5. This device, which will be in operation during 1972,

-13

should produce a documented accuracy near IX 10 , once the complete system is fully evaluated. Present plans call for operation of

this new NBSFS only on an intermittent basis in order to periodically

calibrate the frequencies of the working ensemble of clocks in the NBS

time scale system. These data, as described next, are used in producing the AT(NBS) and other related time scales.

2. 1.2 Computed or "paper" time scales

The Section utilizes six oscillator -clock combinations that produce the computed time scales. One might ask not only what is a "paper" time scale, but also how is it produced, and then how it is

used.

The paper time seales, AT(NBS) and UTC(NBS), are in fact

computer printouts which relate the indicated time of each of the six

clocks in the ensemble to the computed scales that are produced by the

computer. The computer utilizes the current measured or estimated

frequency of each oscillator with respect to the NBSFS, along with the

past history of each of the six oscillators to apply a weighting factor to

each oscillator based on its performance. The computed scales produced are more uniform than a scale produced by the best unit of the

ensemble. The computer output is a set of numbers that are the time

differences between each clock output and the computed scales. With

See Appendix
this information, one has a mechanism for comparing the timekeeping

performance of individual clocks comprising the time scale system.

Figure 1 is a simplified diagram of the system that produces the NBS

time scales.

Z. 1.3 Operational Clock Systems

The Section maintains a number of operational clock systems.

The outputs of these systems are electrical seconds pulses as well as visual displays of time of day. The time scale based on the primary

NBS clock, designated as Clock #7, closely approximates the AT(NBS)

time scale. Corrections for Clock #7 relative to the AT(NBS) time

scale are computed at regular intervals. Probably the most useful

clock in a practical sense is the unit designated Clock #8. This

clock's output closely approximates the UTC(NBS) time scale. The

daily deviations of Clock #8 from the UTC(NBS) time scale are used to compute data found in the NBS Time and Frequency Services Bulletin,

published monthly. The deviation of this clock from the UTC(NBS)

time scale is constrained always to be less than 1 microsecond and

seldom exceeds _+ 0.2 microsecond. Another system, operating from

an independent cesium beam standard, is a backup atomic rate clock

designated Clock #0. A time comparator and twenty-four hour alarm

system are associated with Clock #0 and Clock #7. This alarm is

actuated if Clock #7 diverges from Clock #0 by more than 5 microseconds. An adjunct to the operational clock systems is the clock

comparison link to Fort Collins that utilizes television synchronizing

pulses. Figure 2 is a simplified diagram of these operational clock

systems

*

It should be noted that three of the six atomic clocks in the ensemble

are physically located at Fort Collins, Colorado.
2. 2 Radio Station WWV, Fort Collins, Colorado

The heart of the time and frequency generation system at WWV

is a set of three commercial cesium-beam frequency standards.

These standards are the basis for three identical generating units which

provide to the transmitters a composite rf signal containing the complete

WWV format.

The cesium standards, through a series of dividers and distribution amplifiers, drive the three master clocks, or more specifically,

the three WWV time -code generators. These time -code generators

provide the standard audio tones, time -ticks, and all the gates, codes,

etc., necessary to produce this rather complex format. Figure 3

shows a simplified drawing of the WWV time and frequency generating

system at Fort Collins.

2.3 Radio Station WWVB, Fort Collins, Colorado

The WWVB time and frequency generating system is somewhat

similar to that of WWV, although not as elaborate. WWVB uses a highly stable quartz crystal oscillator as the standard frequency

generator. This crystal oscillator is referenced against NBSFS as noted later. Following this quartz crystal oscillator is a device known

as a frequency drift corrector, which compensates for both frequency

offset and rate change of the quartz crystal oscillator. The format for

the time code and the 60 kHz driving frequency are produced by a special time-code generator, while two other generators are driven by

a second quartz oscillator. These generators, along with the oscillators and other equipment, provide three semi-independent generating

systems. Figure 4 shows the arrangement of this equipment.
2.4 Radio Station WWVL, Fort Collins, Colorado

(Intermittent Experimental)

The frequency generation for WWVL. is like that of WWVB in

that quartz crystal oscillators and drift correctors are used as the

primary frequency generators. Since there is no complex time format

in this case, the one, two, or three operating frequencies are programmed to the transmitter by NBS-built equipment. The synthesizers

are units from commercial VLF phase -tracking receivers. If multiple

frequencies are being transmitted, the transmitter is shut off for about

0.1 second out of each period to allow the frequency changeover. The

carrier shutoff is "on time" with respect to the UTC(NBS) time scale.

See Figure 5 for a simplified diagram of this system. The transmission period of each frequency is usually ten seconds.

The local servo system for the generation and control equipment is discussed in Section 3. Figure 5 shows this system.

2. 5 Radio Station WWVH, Kauai, Hawaii

Station WWVH began operation at an old Navy site on the island

of Maui in 1948. After long and distinguished service, this station was

•V relocated near the town of Kekaha on the island of Kauai. The standards at WWVH are referenced indirectly against NBSFS via the NBS

broadcasts and by way of portable clocks that are referenced to the

U, S. Naval Observatory time scale, which is, in turn, referenced to

the NBS time scale.

The generation system for WWVH, Kauai, Hawaii, is essentially the same as the system at WWV. Figure 3 shows this system.

This transfer taking place at hours UT, July 1, 1971
3. TRANSMISSION OF TIME AND FREQUENCY

3. 1 Transmission from WWV and WWVH

The multiple outputs from the rf driver units in the shielded

control room are supplied directly to the transmitters. The WWV and

WWVH transmitters are simply high-power linear amplifiers and

therefore do not contain audio circuits or modulators. The delay from

the time -code generator to the antenna is much less than the transmitted

accuracy specified for either station and is therefore neglected.

-12

The specifications for WWV are: frequency within + 1 X 10 of

the NBS Frequency Standard; time within _+ 5 microsecond of the

UTC(NBS) time scale. The time and frequency specifications for WWVH

are somewhat degraded due to the difficulty in making comparisons over

such long geographical distances.

3.2 Transmission from WWVB

WWVB's transmitter and radiating system have an overall "Q"

factor of somewhat less than 100. This "Q" factor is sufficiently low

to allow operation without any phase control on the antenna. Future

plans call for a "local" servo system to continuously adjust the transmitted phase at the antenna to be in agreement with the local station

reference. The local servo system compensates for the phase perturbations due to wind shifting the antenna. These excursions seldom

exceed 0. 5 microsecond peak to peak. To correct for discrete phase

shifts that occur whenever the antenna is tuned, a manual phase compensation is made based on a continuously operating phase monitor.

The relationship between a time pulse or carrier cycle and the

marker of a time scale becomes important when one operates a standard time and frequency station in the LF range. The time at which a
particular carrier crossover occurs for WWVB is published on a dailybasis in the monthly issues of the Time and Frequency Services

Bulletin. The first carrier crossover at the antenna occurs about six

microseconds after the marker pulse of UTC(NBS).

The following points should be noted: (1) The radiated phase is

late relative to the time marker of UTC(NBS) because of the delays

through the WWVB transmission system. The master time -code generator at WWVB is maintained in close agreement with UTC(NBS), and

any time difference between UTC(NBS) and the WWVB time -code generator is known at all times. (2) When a phase or time error between

the marker of UTC(NBS) and the WWVB time -code generator, and

hence the WWVB radiated phase, occurs, the error is corrected by

changing the frequency of the WWVB quartz crystal oscillator. The

maximum rate of correction is limited to 0.1 microsecond per fourhour period. In other words, during times of phase correction, the

frequency of WWVB can be in error with respect to the NBS reference

frequency by as much as 6 X 10

3. 3 Transmission from WWVL (Experimental)

The transmitted phase of WWVL is controlled more precisely

than that of any other NBS transmission. These are two reasons for

this: (1) The realizable stability of the transmission through the medium in the 20 kHz region is quite high; and (2) the susceptibility of the

transmission system to phase perturbations is also quite high. The "Q"

of the antenna system is of the order of 1000. With a tuning reactance

of greater than 500 ohms, a change in this reactance of 0.2 percent

causes a phase shift of 45° or more than 6 microseconds. Every effort

is made to hold the transmitted phase to within + 0.1 microsecond of its

nominal value. This calls for a highly sensitive "local" servo system.

10
This local servo begins with a pickup coil, located in the

building containing the antenna loading coil. The voltage from this coil

is supplied through a buried coaxial cable to the shielded control room

in the transmitter building. In the control equipment, this signal is

compared with the rf output of the WWVL synthesizer. Any phase shifts

detected at the loading coil building are quickly compensated for by the

servo -driven phase shifter. Each transmitted frequency has its own

phase shifter, but the system contains a single servo motor geared to

all the phase shifters. Since each phase shifter is continuously driven,

any phase error is "precorrected" to the phase shifter that will be controlling the transmitter during-the upcoming transmitting period. This

precorrection reduces the phase noise of the transmission considerably.

In addition, the input phase to the transmitter is adjusted so that the

zero voltage crossover of the radiated field as measured at the antenna

occurs in coincidence with the time marker of the UTC(NBS) time scale.

Again, -as in the case of WWVB, any errors between the radiated

phase and the time marker of UTC(NBS) are known at all times and corrected by the same procedure and at the same maximum rate of ~ 6

parts m 10

4. TIME AND FREQUENCY INTERCOMPARISONS

4. 1 WWV and WWVH Self -Comparisons

As we mentioned earlier, WWV and WWVH have three independent time and frequency generating systems: First, the phases of the

1 MHz outputs from the dividers following the cesium standards are

intercompared. The outputs of the phase detectors used are applied to a multi-channel chart recorder with a 1 -microsecond full scale.

Second, following the manual phase shifter, the phases of the 1 MHz

outputs from the next set of dividers are intercompared. The phase

11
comparison record is arranged to be on the same multichannel recorder,

The third inter comparison is accomplished at the time -code generators.

The IRIG-H time code from each generator is monitored by a code

comparator. The alarm is sounded if the output from any unit diverges

from any other by 5 microseconds, or if a clock jumps phase such that

its code is misaligned with respect to that of any other unit. With these

three comparison systems, any standard, divider chain, or time -code

generator that fails can be immediately detected. See Figure 3 and

Figure 6 for simplified block diagrams showing the comparison systems,

4. 2 WWV to WWVB and WWVL

The standard 100 kHz output from the WWV time and frequency

generating system designated "Rack A" is sent by coaxial cable to the

WWVB/VL control room. This standard frequency drives a digital

clock designated as the "Fort Collins Master Clock. " All measurements of the WWVL and WWVB local clocks, which are driven by the

quartz crystal oscillators, are in terms of this Master Clock. This

Master Clock pulse is also returned by coaxial cable to the WWV control room.

This system of clock intercomparisons effectively prevents

undetected clock or time -code generator failures.

In addition to the clock comparisons, the quartz crystal oscillators that form part of the LF and VLF generating systems are continuously compared with the standard 100 kHz signal from WWV (see

Figure 6).

4. 3 Between the Fort Collins Standards and the WWVH Standards

Station WWVH uses all available information to maintain close

ties with the Fort Collins standards. This includes monitoring of

12
stations WWVB and WWVL and frequency comparisons, via portable

clock, with a phase -stabilized Naval communications station on the

island of Oahu. Via portable clock and other means the relationship

between the master clocks at this Naval radio station and the clocks at

Fort Collins and Boulder is known at all times to within a few tenths of

microseconds.

4.4 Between the Fort Collins Standards and the NBS Standards

4.4. 1 Portable Clocks

A reliable high-precision method of comparing the Fort Collins

Master Clock with the NBS/Boulder Clock #8 is to physically carry a cesium-standard-driven clock between the two locations. A highly

accurate clock of this sort normally loses or gains less than 0.1

microsecond during the four -hour round trip. To perform this task on

a daily basis is expensive and unnecessary. Nevertheless, a portable

clock is occasionally carried to Fort Collins when circumstances

dictate.

4.4.2 TV Synchronizing Pulse Method

a. Theory

Because the portable clock method for clock synchronization is

expensive and time consuming, and the Boulder -Fort Collins continuous phase loop had been eliminated because it had short-term phase

errors due to sky-wave interference, a new method had to be found.

In 1967, Tolman, et al described a method for comparing remote

clocks using television synchronizing pulses for time transfer. Since

1 Tolman, et. al, "Microsecond clock comparison by means of TV

synchronizing pulses, " IEEE Trans, on Instr. and Meas., Vol. IM-16,

No. 3, September, 1967.

13
May, 19^8, this method has been used to compare the Master UTC

Clock at Fort Collins with the NBS Clock #8 at Boulder.

Figure 7 is a space -time diagram of how the method is

developed. Assume Figure 7 is a map of the geographical area in

question at one instant in time. Located on the map are five TV synchronizing pulses as they might be at this instant.

From the diagram, if the Boulder clock and the Fort Collins

clock agree, the delay time over the distance D - D is

4 A D = + B - A microseconds, (1) c

where — = one synchronizing pulse period, or 63.55 microseconds,

c The values B and A are found by using the local clock tick as the start

pulse of a time -interval measurement and a synchronizing pulse from

a local TV set as the stop pulse. In the case of Figure 7, pulse #1

stops the Boulder time interval measurement at the value "A" and

pulse #5 stops the Fort Collins time interval measurement at the value

"B". If now the Fort Collins clock is early relative to the Boulder

clock, the value B will increase because the interval will be longer.

From the diagram, this longer interval B 1 will be

B' = B + At (2) e

where At is the amount early.

e

' Computing the delay time D using great circle distance calculations and substituting eq. (2) into eq. (1),

4 A At = - D + (B 1 - A) microseconds . (3) e c

14
b. Practice

The TV synchronizing pulse method of clock intercomparisons

was put in service in May, 1968. The measurements have been made

on a daily basis since then. During the actual measurement, some ten

readings are taken. The spread over this number of readings seldom

> exceeds _+ 0.5 microsecond. The average of this group is recorded.

When the Fort Collins Master Clock has diverged from NBS Clock #8

by about one -half microsecond, it is reset to the correct time.

5. APPENDIX

5. 1 Explanation of NBS Time Scales

AT (NBS)

AT (NBS) is an Atomic Time Scale, previously called NBS -A,

whose rate is determined by the primary frequency standard of the

National Bureau of Standards (NBSFS). This standard realizes the

second as defined in the International System of Units (SI). The origin

marker of the scale was in agreement with UTC(NBS) at 0000 UT

1 January 1958.

UTC(NBS)

UTC(NBS) is a coordinated time scale; i.e., the BIH determines

when steps of 1.0 second should occur to keep this Universal Time

Scale in approximate agreement with UT1. The NBS and the USNO

agreed on a coordinate rate for these two time scales starting

0000 UT1, October, 1968. The coordinate rate was chosen as an

average of the two rates, as determined by portable clocks, prior to

1 October 1968. The USNO and NBS intend to maintain synchronization

of the UTC scales to within about 5 microseconds by using an appropriate coordinate rate.

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22
FORM NBS-114A 11-71) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS TN-379-12. Gov t Accession No.3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Standard Time and Frequency: Its Generation, Control, and

Dissemination From the National Bureau of Standards Time

and Frequency Division

5. Publication Date June 1972

6. Performing Organization Code

7. AUTHOR(S) John B. Milton8. Performing Organization

10. Project/Task/Work Unit No. 2730128

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS, Boulder Labs,

DEPARTMENT OF COMMERCE

Boulder, Colorado 803 02

11. Contract/Grant No.

12. Sponsoring Organization Name and Address National Bureau of Standards

Boulder, Colorado 80302

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) The Time and Frequency Division of the National Bureau of Standards produces the

NBS time scales, AT(NBS) and UTC(NBS). These time scales are developed by

utilizing the properties of the NBS frequency standard, NBSFS. The main byproduct

of these time scales is the operational clock systems. These operational clock

systems are used, among other things, to calibrate the clocks and secondary standards necessary for the operation of the NBS radio stations, WWV, WWVB, WWVL,

and WWVH. These stations transmit UTC(NBS), and various tones, alerts, and

corrections for time -of -day information.

1'. KEY WORDS (Alphabetical order, separated by semicolons)

clock synchronization; frequency and time dissemination; primary frequency standard;

standard frequency broadcasts; time interval; time scales

18. AVAILABILITY STATEMENT

P^l UNLIMITED.

I I FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

26

22. Price

. 35

GPO 843 - 002 USCOMM-DC 66244-P71

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